The invention relates to a geophone which varies the optical path length of an optical fiber in response to a seismic signal for use in seismic exploration equipment. A seismic signal in this application refers to an acceleration or velocity signal. The geophone of this invention may be included within a sensor array which is formed as a string of transducer assemblies.
A conventional geophone uses a permanent magnet and electric coils suspended by springs or other means such as air, oil, and the like, to dampen oscillations of a mass within the geophone. A number of geophones are connected to a seismic cable, which ultimately is connected to a recording vehicle, such as a truck. A seismic signal is generated by way of an explosion or other vibration source. The vibrations create motion of the geophone which in turn causes the coil to move. The movement of the coil in the magnetic field induces a flow of electric current which is detected and recorded at the seismic truck.
The dynamic quantity measured by a geophone depends on the method for detecting the position of the mass, relative to the casing. Using electromagnetic induction in a pick-up coil gives a velocity output. A displacement geophone uses a detection method that measures the position of the mass directly and therefore gives an output that is proportional to casing displacement.
Fiber optic sensors that respond to variations in acceleration or velocity have shown increasing promise as geophones. Known means for measuring such variations include interferometers which detect changes in optical phase and spectrometers which detect spectral shifts. For example, certain fiber optic interferometric sensors respond to an acceleration or velocity signal developed from the reflection of a signal from a seismic source by varying the effective length of the fiber optic filament in response to the perturbation.
In such applications, optical fibers are made sensitive to acceleration or velocity of the sensor. An optical fiber exposed to such phenomena changes the medium through which a light or infrared beam passes that is guided by the fiber. Optical fibers have been considered for use as sensing elements and devices such as microphones, hydrophones, magnetometers, accelerometers, and electric current sensors, and particularly for geophones.
However, developers of such systems continue to seek a simple, inexpensive, yet sensitive geophones which can endure the often harsh conditions to which they are subjected. There remains a need for a such a geophone that is responsive to variations in displacement, velocity, or acceleration, using variations in the stress on a fiber optic element. To be commercially successful, such a geophone must be both robust and easily manufactured.